1. Field of the Invention
The present invention relates to a mask for silicon crystallization, a method for crystallizing silicon using the same and a display device. More particularly, the present invention relates to a mask for silicon crystallization, wherein the number of grain boundaries of crystallized silicon can be minimized, a method for crystallizing silicon using the mask and a display device.
2. Description of the Related Art
In a liquid crystal display (LCD), the amount of light transmitted through the liquid crystal (LC) panel is adjusted according to an image signal. The image signal is applied to a plurality of control switches arranged in a matrix, so that a desired image can be displayed on the LCD panel. An LCD is classified into an amorphous silicon thin film transistor (TFT) LCD and a polysilicon TFT LCD. A polysilicon TFT exhibits superior device characteristics to an amorphous silicon TFT, and a driving circuit can be mounted on a substrate of the polysilicon TFT. A crystallizing method for obtaining such a polysilicon thin film includes a variety of methods such as solid phase crystallization (SPC), rapid thermal annealing (RTA), excimer laser annealing (ELA) and sequential lateral solidification (SLS).
FIG. 1A is a schematic illustration of a sequential lateral solidification (hereinafter, referred to as “SLS”) technique, and FIG. 1B is a schematic view of silicon crystallized by the SLS technique.
The SLS technique is a technique for crystallizing amorphous silicon after locally melting the amorphous silicon by using a slit formed in a mask. As compared with the existing ELA technique, the SLS technique has several advantages. For example, a variety of particle sizes (a few μm to a single crystal) can be achieved as desired, process margin is increased, and productivity is greatly enhanced because there is no limitation for the substrate size and no vacuum is required. Accordingly, much attention to the SLS technique has been paid as the next-generation crystallization technique. As shown in FIG. 1A, a laser beam passes through a slit 15 formed in a mask 10, melting the amorphous silicon locally. The slit is about a few μm long. A melted region 25 of a substrate 20 crystallizes as it is cooled, such that the crystals grow from the boundary between the melted region 25 and a neighboring unmelted region 27. The crystals grow toward the center of the melted region 25 and stops growing when particles meet one another at the center. The aforementioned process is repeated while moving the slit 15 little by little over the substrate 20, eventually crystallizing the entire substrate 20.
FIG. 1B shows a state where amorphous silicon has been crystallized using a straight slit. In this figure, the arrows designate crystal growth directions. When using such an SLS technique, the shape and size of a particle formed can be changed depending on the shape of the slit.
FIG. 2 schematically illustrates the process of a general single scan two-shot SLS. Referring to FIG. 2, when a shot of laser beam (first shot) is directed onto an amorphous silicon thin film using a mask with a plurality of slits, melted portions 25a and 25b are formed. The melted portions crystallize as they cool. The entire substrate can be crystallized by repeating the process of irradiating portions of the substrate with a laser beam (by using a second shot, third shot , . . . , and n-th shot) while moving a mask over the substrate between the shots.
FIGS. 3A and 3B are graphs showing the characteristics of a TFT that uses the silicon crystallized by the SLS process of FIG. 2. FIG. 3A is a graph plotting the characteristics of a TFT with a TFT channel formed in a horizontal direction, i.e. a crystal growth direction, and FIG. 3B is a graph plotting the characteristic of a TFT with a TFT channel formed in a vertical direction. A particle size nearly corresponding to the slit size is obtained in the horizontal direction (crystal growth direction), while a small particle size corresponding to about a few thousands angstroms (Å) is obtained in the vertical direction.
Referring to FIGS. 3A and 3B, the characteristics of a TFT, e.g. Ion (for Vds=10.1 and Vgs=20) and electron mobility (for Vds=10.1), are shown in the following Table 1.
TABLE 1CharacteristicsDirectionIon (μA)Electron Mobility (cm2/Vs)Horizontal Direction750~900100~120Vertical Direction200~330~30
As illustrated in Table 1, the Ion (μA) and electron mobility (cm2/Vs) in a horizontal direction are about 750 to 900 and about 100 to 120, respectively. Further, the Ion (μA) and electron mobility (cm2/Vs) in a vertical direction are about 200 to 330 and up to about 30, respectively. That is, it can be understood that the horizontal characteristics are markedly better than the vertical characteristics. Due to such directional anisotropy, a TFT channel should be designed in only one direction when a circuit for a system-on-glass (SOG) product is built in a panel. This is an undesirable limitation. An SLS technique in which a two-shot SLS process is performed twice, once in a horizontal direction and another time in a vertical direction, has been conceived to overcome the limitation. According to the SLS technique, at least theoretically, the growth can be achieved in both horizontal and vertical directions. That is, a uniform microstructure with no anisotropic property in the horizontal and vertical directions can be obtained after the crystallization, and thus, the uniform characteristics can also be obtained.
FIGS. 4A and 4B are a view and photograph showing a microstructure of silicone crystallized using only a horizontal silt, respectively; FIGS. 4C and 4D are a view and photograph showing a microstructure of silicone crystallized using both horizontal and vertical silts, respectively.
Referring to FIGS. 4A to 4D, if a mask including both horizontal and vertical slits is used, a particle grown through one slit becomes a seed and grows perpendicularly to the direction of a particle grown through the next slit. However, if the vertical slit does not precisely align with one row of particles between horizontal grain boundaries but simultaneously aligns with parts of two rows of particles as shown in FIGS. 4A and 4C, the problem of anisotropic property in the particle is not completely solved since a sub grain boundary forms perpendicularly to the original grain boundary, as indicated by the circles in FIG. 4C. Since an actual grain boundary is almost never a perfectly straight line, such a phenomenon occurs frequently. Accordingly, the existing 2+2 shot SLS process is not a great improvement relative to the two-shot SLS process, at least from the perspective of limitations imposed by the anisotropic property.